Step 6.18: Implement auto scroll

src/pages/messages/messages.ts

@@ -1,4 +1,4 @@
-import { Component, OnInit } from '@angular/core';
+import { Component, OnInit, OnDestroy, ElementRef } from '@angular/core';
 import { NavParams } from 'ionic-angular';
 import { Chat, Message, MessageType } from 'api/models';
 import { Observable } from 'rxjs';
@@ -9,20 +9,39 @@ import { MeteorObservable } from 'meteor-rxjs';
   selector: 'messages-page',
   templateUrl: 'messages.html'
 })
-export class MessagesPage implements OnInit {
+export class MessagesPage implements OnInit, OnDestroy {
   selectedChat: Chat;
   title: string;
   picture: string;
   messages: Observable<Message[]>;
   message: string = '';
+  autoScroller: MutationObserver;
+  scrollOffset = 0;
 
-  constructor(navParams: NavParams) {
+  constructor(
+    navParams: NavParams,
+    private el: ElementRef
+  ) {
     this.selectedChat = <Chat>navParams.get('chat');
     this.title = this.selectedChat.title;
     this.picture = this.selectedChat.picture;
   }
 
+  private get messagesPageContent(): Element {
+    return this.el.nativeElement.querySelector('.messages-page-content');
+  }
+
+  private get messagesList(): Element {
+    return this.messagesPageContent.querySelector('.messages');
+  }
+
+  private get scroller(): Element {
+    return this.messagesList.querySelector('.scroll-content');
+  }
+
   ngOnInit() {
+    this.autoScroller = this.autoScroll();
+
     let isEven = false;
 
     this.messages = Messages.find(
@@ -38,8 +57,30 @@ export class MessagesPage implements OnInit {
     });
   }
 
+  ngOnDestroy() {
+    this.autoScroller.disconnect();
+  }
+
+  autoScroll(): MutationObserver {
+    const autoScroller = new MutationObserver(this.scrollDown.bind(this));
+
+    autoScroller.observe(this.messagesList, {
+      childList: true,
+      subtree: true
+    });
+
+    return autoScroller;
+  }
+
+  scrollDown(): void {
+    // Scroll down and apply specified offset
+    this.scroller.scrollTop = this.scroller.scrollHeight - this.scrollOffset;
+    // Zero offset for next invocation
+    this.scrollOffset = 0;
+  }
+
   onInputKeypress({ keyCode }: KeyboardEvent): void {
-    if (keyCode.charCode === 13) {
+    if (keyCode === 13) {
       this.sendTextMessage();
     }
   }